Conventionally, in order to enable a high-speed data communication via FTTH (Fiber to the Home), that is in order to send/receive information to/from a home or office with a high-speed wide band, an optical fiber composed of a plastic coated optical fiber, a tight-buffered optical fiber or an optical ribbon fiber is drawn into a subscriber home such as a building or conventional home, from an optical fiber cable for access extended from a telephone exchange. An optical fiber cable for drop is employed to distribute an optical fiber. The optical fiber cable for drop is employed when an optical fiber is drawn into a home or office from a branch closure of a trunk cable supported by a telephone pole or when an optical fiber is drawn into each room within the home or office. As the optical fiber cable for drop, an optical fiber drop cable (drop wire), a little core optical aerial cable and an indoor drop cable are mainly cited. The little core optical aerial cable is an optical fiber cable for drop of which a size of support wire is increased to be applied to a longer laying span length. The indoor drop cable is an optical fiber cable for drop to be employed when an optical fiber is drawn into each room within a home or office.
As shown in FIG. 1, a conventional indoor drop cable 101 includes an elongated optical element portion 115. The optical element portion 115 is composed of an optical fiber 103, at least a pair of tensile strength members 105 and 105, an outer jacket 107, and notches 113 and 113. The optical fiber 103 is composed of a plastic coated optical fiber, a tight-buffered optical fiber or an optical ribbon fiber. The tensile strength members 105 and 105 are arranged in parallel at both sides of the optical fiber 103 in a width direction of the optical fiber 103. The outer jacket 107 is made up of a resin, covers outer circumferences of the optical fiber 103 and the tensile strength members 105 and 105, and has a rectangular cross-sectional shape. The outer jacket 107 has a long side arranged in the width direction of the optical fiber 103 and a short side arranged in a thickness direction of the optical fiber perpendicular to the width direction. Viewed from the cross-sectional surface of the outer jacket 107, the notches 113 and 113 are formed on a surface of the outer jacket 107 such that they lie at both sides of the optical fiber 103 on a Y-axis 111 which passes the center of the optical fiber 103 and is perpendicular to an X-axis 109 connecting the centers of the optical fiber 103 and the tensile strength members 105 and 105.
Generally, the tensile strength member is a steel wire having a diameter of 0.4 mm. As an outer jacket material of the outer jacket 109, a compound of a non halogen fire-retardant such as magnesium hydroxide or aluminum hydroxide and a based polymer such as ethylene-vinyl acetate copolymer (EVA resin) or ethylene-ethyl acrylate (EEA) has been employed. The outer jacket has an outer diameter composed of a short side of about 2.0 mm and a long side of about 3.0 mm.
In order to efficiently insert an optical fiber cable into a pipe such as an electric conduit where an electric cable or the like has been already laid, there is devisal to define the bending rigidity of optical fiber cable to increase insertability of the optical fiber cable into the pipe.
For example, a patent document 1 discloses an optical fiber for air pressure feed of which the bending rigidity is defined within a range of 300 N·mm2 to 400 N·mm2. A patent document 2 discloses an optical fiber cable having a function similar to an insert tool, of which the bending rigidity is defined within a range of 0.06 N·m2 to 0.12 N·m2. A patent document 3 discloses an optical fiber drop cable of which the bending rigidity is defined within a range of 80 N·mm2 to 500 N·mm2.